1. Field of the Invention
This invention relates to an image sensor, and more particularly to an image sensor having microlenses of at least two different radii of curvature, and to an image sensor having an asymmetrical microlens.
2. Description of the Related Art
Digital imaging devices have been widely used in many electronic products nowadays. They are used in, for example, digital cameras, digital video recorders, cellular phones with photographing function, safety-control monitors, etc.
A digital imaging device usually includes an image sensor chip, such as a CCD image sensor chip or a CMOS image sensor chip. For better optical performance, an image sensor chip usually includes a layer of multiple microlenses, so that incident light may better focus on a focal plane, i.e., within a photodiode layer. The photodiode layer receives photons and generates electrical signals thereby.
FIG. 1 shows a cross-sectional view of a conventional image sensor. As shown in FIG. 1, the structure includes a bottom substrate 11, a photodiode layer 12, an interconnection layer 13 (shown as one metal layer for simplicity, but may include multiple metal layers), a passivation layer 14, a color filter layer 15 which includes multiple red (15R), green (15G) and blue (not shown) segments, a spacer layer 16, and a microlens layer 17 which includes multiple microlenses 171 for focusing incident light onto the interface between the photodiode layer 12 and the substrate 11. Layers above the microlens layer 17, such as lens, package and bond pad layers, etc., are omitted for simplicity.
The conventional method for making such an image sensor with microlenses, is to first form a semi-finished substrate with layers 11-16 by conventional semiconductor process steps, and then coat a photoresist layer on the layer 16. The photoresist layer is exposed according to a pattern on a photomask, and developed to form multiple square segments 172 as shown in FIG. 2. Thereafter, a reflow step is taken, that is, the semi-finished substrate with the photoresist layer thereon is subject to a temperature of above 150 degree centigrade for 10 minutes, so that the photoresist layer is partially melted; due to viscosity of the photoresist material, the melted photoresist layer has the contour as the microlenses 171 shown in FIG. 1. Next, the substrate is cooled down to form solid microlenses 171.
The above-mentioned conventional image sensor has the following drawback. The microlenses 171 are all formed of the same radius of curvature. However, light projected onto microlenses at different locations, in particular in an image sensor for use in a medium to large size digital imaging device (mega pixels or above), may have different incident angles. More specifically, as shown in FIG. 3, light vertically projected onto the microlenses at the center area is received by the microlenses at the peripheral area with a tilt angle, causing a vertical shift of focus. The spot sizes 181 at the peripheral area are not satisfactory, and the sensitivity of the image sensor is reduced.
U.S. Pat. No. 6,417,022 discloses a method for producing a microlens with a long focal length, to cope with thick metal layer total thickness due to increased number of metal layers. However, in this cited patent, all of the microlenses on a chip are of the same radius of curvature. This cited patent does not describe any solution to the above-mentioned drawback shown in FIG. 3.
In view of the above, it is desired to provide an image sensor with better sensitivity, wherein the radii of curvature of microlenses at different locations are designed in correspondence with different optical requirements.